Pollution knows no borders. When pollutants are lofted into the atmosphere, strong winds in the region of the upper troposphere and lower stratosphere can carry the noxious chemicals around the globe before they have a chance to decay into more benign forms.

Researchers know that during the boreal summer, the Asian summer monsoon (ASM) plays an outsized role in lifting pollutants up into this boundary region between the troposphere and the stratosphere. Given that recent trends of explosive growth in both population and emissions in Asia are expected to continue, understanding exactly how the ASM affects the distribution of pollutants in the atmosphere will become ever more important.

Here Santee et al. used measurements from an instrument aboard NASA’s Aura satellite, which launched in 2004, to examine trace gases and cloud ice water content in the boundary region between the upper troposphere and the lower stratosphere throughout the life cycle of the ASM anticyclone, a region of high pressure around which winds circulate. The team used data collected by the Microwave Limb Sounder aboard the satellite to quantify water vapor, carbon monoxide, chloromethane, acetonitrile, and methanol as tracers of tropospheric air, as well as ozone, nitric acid, and hydrochloric acid as tracers of stratospheric air, as the ASM anticyclone developed and dissipated. Previous studies have used MLS data to study the effect of the ASM on atmospheric composition over limited time periods or altitude regions, but this study looks at 10 years of MLS data from about 10 to 18 kilometers in altitude using the latest version of MLS data processing software, which can better quantify these trace gases in the presence of thick clouds than previous versions.

By combining MLS data with meteorological analyses from NASA’s Goddard Earth Observing System, Version 5.9.1, the team provided a comprehensive overview of the ASM’s impact on the entire evolution of the convective system from April to October. Each year, as the anticyclone spins up in April, the MLS observes undisturbed background levels of the trace gases, but as the anticyclone strengthens, the gradient of tracers between the edge of the weather system and the surrounding air grows steeper. The team found that abundances of many of the pollutants, including carbon monoxide, chloromethane, acetonitrile, and methanol, peak in June and July, after the monsoon reaches maturity, attaining heights as high as 20 kilometers.

The study provides a benchmark for the effect of the ASM on atmospheric composition, but future work will need to resolve the effects of interannual and intraseasonal variability in both the anticyclone and the characteristics of the boundary between the troposphere and the stratosphere. This work lays the groundwork for future studies of this weather system: an important step as anthropogenic emissions continue to reshape the global climate. (Journal of Geophysical Research: Atmospheres,https://doi.org/10.1002/2016JD026408, 2017)

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